This invention relates in general to the field of three-dimensional (3D) volumetric displays. More specifically, it relates to data writing schemes to increase resolution of volumetric 3D displays using projectors as image sources.
Tsao U.S. Pat. No. 6,302,542 B1 describes a typical type of V3D display applying the Moving Screen Projection technique. As shown in FIG. 7, the system comprises three major portions:
(1) The moving screen: The periodically moving screen 11 displays the projected images and forms the display space 12.
(2) The image delivery mechanism: The image delivery mechanism 13 relays the projection beam 14, which contains a set of 2D image frames 14a, projected from the image projector onto the moving screen for displaying. The mechanism keeps the size and focus of the projected image invariant as the screen moves, and also keeps the orientation of the projected image frame synchronous to the moving screen. (Because direct whole-frame projection from the projector onto the screen is not possible due to the constant motion of the screen.)
(3) The image projector: The image projector 15 generates and projects the set of 2D image frames 14a, through the image delivery mechanism 13, onto the moving screen 11. The image projector generally consists of an image generating panel, a projection lens, and a light source.
By sweeping the screen across the space periodically and rapidly, and sequentially project onto the screen a series of 2D image frames, e.g. the profiles of an auto-body 14a in FIG. 7, the set of 2D image frames can thereby be distributed and displayed over the display space, with each frame located at a specific position in the space. This set of 2D image frames, when viewed from outside the display space, forms a 3D volumetric image, because of the after-image effect (persistence of vision) of human eyes. The image actually occupies space and can be viewed from different angles by many viewers simultaneously without the need of any glasses.
The screen motion can be rotating, reciprocating, or rotary-reciprocating. Correspondingly, the image delivery system can be a set of reflectors with rotating motion, reciprocating motion or rotary-reciprocating motion. Details and variations of the system are described in related patents Tsao et al., U.S. Pat. No. 5,754,147; Tsao, U.S. Pat. No. 5,954,414; and Tsao, U.S. Pat. No. 6,765,566 B1, which are incorporated herein by reference.
In the above example of V3D display, the preferred image source for the projector is a spatial light modulator (SLM) such as DMD (Digital Micro-mirror Device) or FLCD (Ferroelectric Liquid Crystal Display). These are devices of black and white pixels. Using a single DMD or FLCD with a white or monochrome light results in a monochrome volumetric 3D display. To create colors, one can use three DMDs or FLCDs, each illuminated by light of a different primary color. Alternatively, Tsao U.S. patent application Ser. No. 09/882,826 describes a method of using a single panel to generate colors. The single panel is divided into three sub-panels and each sub-panel is illuminated by light of a different primary color. The images of the three sub-panels are then recombined into one at projection. (See FIG. 10a of the application as example)
In general, the resolution of a SLM-based V3D display in the direction of screen motion is limited by the frame rate of the SLM. However, a “Sub-frame Method” can be applied to redistribute the pixels on the SLM in the direction of screen motion and to increase the effective frame rate of the SLM. Tsao U.S. Pat. No. 6,765,566 B1 describes the method. The method first separates each color image frame from a projector into three sub-frames in the time domain, each sub-frame of a different primary color, by modulating and switching the color of the illumination source with a color switching means or switching after the projection lens. In the case when the sub-frames are from an SLM defined with sub-panels, this separation is achieved by modulating the illumination to each sub-panel such that only one sub-panel is illuminated at any time and for only a short period of time, as described in Tsao U.S. patent application Ser. No. 09/882,826. Then projected images from three such projectors are superimposed in a manner such that at any moment a superimposed image frames contains 3 sub-frames, each from a different projector and of a different primary color. This allows the display of color V3D images of resolution higher than allowed if only full frames, instead of sub-frames, are used. It will be nice if the need for illumination modulation can be removed in the method of Sub-frame Method.